The present invention relates to a valve spring retainer used in a valve train of an internal combustion engine, and particularly to a valve spring retainer produced from metal sheet and a method of manufacturing it.
The valve spring retainer made of metal sheet is lightened to reduce inertial mass of the valve train thereby improving the performance of the internal combustion engine in JP55-17976A, JP62-185807A and JP2002-303107A.
In the valve spring retainer made of metal sheet, a steel sheet is molded by a press and an original of the valve spring retainer is formed. In order that wear resistance is improved, its surface is hardened The original is generally treated with heat such as carburizing, quenching and tempering.
By heat treatment, a carburized layer is formed on the surface of the valve spring retainer. At the same time, carburizing is carried out in an ordinary carburizing furnace other than a vacuum carburizing furnace, so that an oxide film is formed on the surface of the carburized layer. The oxide film provides hardness, so that a spring-retaining flange in the valve spring retainer pressed by the upper end of the valve spring is improved in wear resistance.
However, the spring-retaining flange is always in contact with the end face of the valve spring during running of the engine. When the valve spring is compressed, bending stress is exerted to the spring-retaining flange, which is flexed. The spring-retaining flange relatively slides on the valve spring. The oxide film formed on the part of the valve-retaining flange pressed by the upper end of the valve spring peels off. By grinding effect of the peeled oxide, the contact surface between the spring-retaining flange and the valve spring can be worn. The thicker the oxide film is, the more likely the problem occurs.
Particularly, the valve spring retainer made of metal sheet is thin, so that it is necessary to prevent wear of the spring-retaining flange as much as possible.